Implement native Iterator system

Description

Currently, Vx for loops are hardcoded to iterate over integer ranges (for i in start..end { ... }). We need native support for generalized iterators, similar to Rust's Iterator trait, to allow iterating over custom collections, arrays, and ranges seamlessly.

Proposed Implementation

1. Parser & AST Extensions:
   • Add support for generic parameters in traits (e.g., trait Iterator<T> { ... }).
   • Add support for generic parameters in impl blocks (e.g., impl<T> Iterator<T> for MyStruct<T>).
2. Standard Library Additions:
   • Introduce enum Option<T> { Some(T), None }.
   • Introduce trait Iterator<T> { fn next() -> Option<T>; } (or with appropriate self binding once supported).
   • Introduce a Range struct that implements Iterator.
3. Compiler Lowering:
   • Update the ForLoopStmt parser to accept any expression (for item in expr { ... }).
   • Modify the semantic analyzer to verify that expr implements Iterator or IntoIterator.
   • Lower the for loop in MLIR into a generic loop that calls .next() and matches on Some/None.

Start by tracing the ForLoopStmt parser, semantic analyzer, and MLIR lowering mentioned in the proposal. Review how integer ranges are currently represented and lowered, then map the required parser, AST, standard-library, type-checking, and lowering changes. Done means custom collections, arrays, and ranges can be iterated through the native Iterator and Option abstractions.